In an active matrix substrate, a thin film transistor (hereinafter also referred to as a “TFT”), for example, is provided as a switching device in each pixel, which is a minimum unit of an image.
For a recent active matrix substrate, as a switching device as a minimum unit of an image, a TFT using a semiconductor layer of an oxide semiconductor (hereinafter also referred to as an oxide semiconductor layer) formed by an IGZO (In—Ga—Zn—O)-based oxide semiconductor film having high mobility has been proposed as a replacement of a conventional thin film transistor using a semiconductor layer of amorphous silicon.
More specifically, a TFT having a double gate structure including, for example, a first gate electrode provided on the insulating substrate, a first gate insulating film covering the first gate electrode, an oxide semiconductor layer provided on a first gate insulating film and overlapping the first gate electrode, source and drain electrodes connected to the oxide semiconductor layer, a second gate insulating film covering the oxide semiconductor layer, and a second gate electrode provided on the second gate insulating film, is proposed (see, for example, Patent Document 1).